This invention relates generally to an integrated circuit including a resistor string and to a method for its manufacture, and more specifically to a resistor string integrated circuit having reduced linearity error and to a method for reducing the linearity error of a resistor string integrated circuit.
Some integrated circuits (ICs) such as digital to analog converters (DACs) and analog to digital (ADCs) converters use a resistor string to provide a plurality of comparator reference voltages. The resistor string includes a plurality of resistors, ideally identical unit resistors, coupled in series between two external reference voltages, a high reference voltage (VrH) and a low reference voltage (VrL). To implement an n-bit DAC, a string of 2n resistors is required. If the resistors are identical, by contacting the nodes between the 2n resistors, 2n+1 precisely known reference tap voltages (including the two external reference voltages) can be realized. In an 8-bit DAC, for example, 28 or 256 resistors are coupled in series to provide the DAC output voltages. Depending on the DAC input code, one of these tap voltages is directed to the output of the resistor string.
Unfortunately, it is not possible to produce a large number of identical resistors on an integrated circuit chip. Because of this, the 2nxe2x88x921 internal tap voltages cannot be precisely known. Errors in resistor values, such as random and gradient errors, can accumulate as one moves from resistor to resistor along the resistor string between the two external reference voltages. This cumulative error results in an integral non-linearity (INL) error that, in turn, causes error in the output of the DAC.
Random errors in resistor values are caused, for example, by variations in the physical size of the resistors or by variations in the conductivity of the material from which the resistors are fabricated. These variations may be random or may be caused by gradients in processing variables. Gradient errors (which can be either linear or non-linear) are often addressed in IC layout and manufacture by interdigitating the affected components. With the large number of resistors commonly used in a resistor string, however, it is not practical to interdigitate all of the resistors. For practical layout reasons and to achieve some degree of interdigitation, resistor strings are often laid out in a plurality of columns of series connected resistors and the columns, in turn, are series connected.
For example, FIG. 1 illustrates a prior art implementation of an 8-bit (256 resistor) resistor string 99. Resistor string 99 includes 16 columns 101-116 of series coupled unit resistors. Each column includes 16 resistors such as resistors 121-136 in column 101. Similar resistors are included in each of the other columns although the other resistors are not labeled with reference numbers. The illustrated placement of the columns matches the relative physical layout location of the columns as implemented in an integrated circuit embodiment. For example, column 102 is physically located between columns 101 and 103. Although it is impractical to interdigitize all of the unit resistors to reduce gradient errors that may occur due to gradients in processing the resistors, it is possible to interdigitate the columns.
In accordance with this embodiment, the columns are connected in series by interconnecting column 101 to column 104 by a conductive interconnect 140, column 104 to column 105 by a conductive interconnect 141, column 105 to column 108 by a conductive interconnect 142, column 108 to column 109 by a conductive interconnect 143, column 109 to column 112 by a conductive interconnect 144, column 112 to column 113 by a conductive interconnect 145, column 113 to column 116 by a conductive interconnect 146, column 116 to column 115 by a conductive interconnect 147, column 115 to column 114 by a conductive interconnect 148, column 114 to column 111 by a conductive interconnect 149, column 111 to column 110 by a conductive interconnect 150, column 110 to column 107 by a conductive interconnect 151, column 107 to column 106 by a conductive interconnect 152, column 106 to column 103 by a conductive interconnect 153, and column 103 to column 102 by a conductive interconnect 154. The end of column 101 is also coupled to a voltage reference terminal 156 that can be coupled to an external voltage reference supply (not illustrated) and the end of column 102 can be coupled to a second voltage reference terminal 158 that can be coupled to an external voltage reference supply (not illustrated).
A voltage reference tap point is provided at the node at each end of each of the unit resistors. For example, in column 101 voltage reference tap points 161-177 are provided at the ends of resistors 121-136, respectively. Similar voltage reference tap points are provided (although not labeled with reference numbers) in each of the other columns. Switches 181-197 are coupled to voltage reference tap points 161-177, respectively to selectively couple a voltage reference tap point to the output (not illustrated) of the resistor string circuit. Again, only the switches coupled to the voltage reference tap points in column 101 have been labeled with reference numbers.
Although this resistor string layout may address some of the gradient errors, it does not address the occurrence and effect of random errors. Further, a gradient from one end of the string to the other will create linearity errors. If the layout of the resistor string is one column of unit resistors or a string of sequentially connected columns of resistors, the linear gradient of the unit resistor values will produce a bow shaped linearity error, e.g., with the correct value at the ends coupled to the external reference voltages and maximum error in the center of the string, at the output of the string.
Moreover, it is not practical to trim the resistance of each of the 2n resistors in an integrated resistor string to a precise value. Because of the large number of components already required for the n-bit string, it is also not practical to add additional components (and hence increase the size of the IC) that might otherwise aid in reducing the linearity error. Accordingly, an integrated circuit and method are needed that can reduce linearity errors in an integrated resistor string but will not increase the complexity of processing the IC and will not increase the device count of the IC.
A resistor string integrated circuit and method according to the present invention addresses many of the problems of the prior art. In accordance with various aspects of the present invention, an improved resistor string integrated circuit having reduced linearity error and to a method for reducing the linearity error of a resistor string integrated circuit are provided.
In accordance with an exemplary embodiment of the invention, to reduce the linearity error that results from the processing of an integrated resistor string, the resistor string is implemented by forming a plurality of unit resistors coupled in series between two voltage reference terminals. The series coupled unit resistors are laid out in a plurality of columns, and the columns are intercoupled in groups of columns. The columns included in each of the groups of columns are suitably selected such that the total resistance of each group of columns is substantially the same. By grouping the columns so that each group of columns has substantially the same resistance, the linearity error is substantially zero at the ends of each of the groups and the linearity error of the entire string is reduced.
In addition, the resistor string integrated circuit and method can be configured with any number of resistors, columns and groups of columns. For example, the linearity of an n-bit integrated resistor string can be improved by arraying the 2n resistors in 2k columns of 2m series coupled resistors each. The 2k columns of resistors are grouped in 2g groups of serially coupled columns of resistors with the total resistance of each group of columns of resistors substantially the same as the total resistance of each of the other groups of columns of resistors.